How do engineers identify damage caused by foundation movement? 

Professional Engineers usually make judgments concerning whether cracks and other forms of damage or distress are due to foundation movement using their knowledge of structural mechanics, the engineering characteristics of building materials and their experience in inspecting hundreds and sometimes tens of thousands of homes.

There are two points that should be recognized at the outset.  If the damage is minor, such as a few hairline cracks, it is very difficult to say definitively what caused the damage.  On the other hand, if the damage is severe, such as,  say, ¼^th wide cracks in the drywall, the damage is almost certainly due to structural distortion.  The problem is with the houses in between.  The following are some rules of thumb published by the Institution of Civil Engineers and the Building Research Department in the United Kingdom.

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What are foundation inspections and foundation performance evaluations? 

A foundation inspection consists of observing the interior and exterior of the house for signs of structural distortion that might be related to foundation movement.  An engineering evaluation of the performance of a foundation  consists of taking the data from the inspection and using it, in conjunction with the engineer’s knowledge of structural mechanics, the structural behavior of houses (including the structural behavior of brick veneer walls, stucco walls, drywall walls and door frames) and the engineering properties of building materials to make engineering judgments about the performance of the foundation.

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Are there different types of foundation engineering evaluations? 

The Texas Section of the American Society of Civil Engineers (TSASCE) has recognized different levels of residential foundation evaluations including a Level A and a Level B evaluation.  The Level A evaluation is usually  referred to as visual evaluation or a report of “first impressions”.  I prefer to describe it as a visible damage evaluation.  The Level B evaluation is built on a Level A evaluation but also includes a finish floor elevation survey.  Some engineers claim to be able to use an elevation survey to confirm or deny whether the observed damage from the Level A evaluation is due to foundation movement.

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How reliable are foundation performance evaluations? 

This is a very interesting question, but it is not clear what the answer is.  In fact it is not clear that it is even possible to answer the question.  We really do not know how reliable foundation performance evaluations are and, I would argue, it is not possible to know how reliable they are.  It is not possible to know, at least in any verifiable, quantitative sense.  Foundation performance evaluations are always subjective opinions.  The subjectivity makes many engineers and some real estate inspectors uncomfortable; but there is no way around the fact that these evaluations are full of subjective assessments and opinions.  For instance any recommendation to underpin or not to underpin a foundation rests, at least in part, on a subjective assessment of the likely effectiveness of underpinning and it's associated risks for a specific foundation.  For a specific foundation, there is simply no way to know how effective foundation underpinning will be and what the costs (in terms of damage to the foundation and the house)  of the underpinning process are until after the foundation is underpinned.  In fact,  it will normally be some time after the repair work before a reliable assessment can be made of how effective the foundation repair was.  And, if the foundation is underpinned, we will never know how the foundation would have performed without the repair.

In a sense deciding to repair or not to repair a foundation is like a fork in Robert Frost's road in his famous poem The Road Not Taken.  There is simply no way to ever know for sure if you made the best decision or not.  For precisely that reason it is important to gather as much information as possible and come to the best understanding you can before making a decision as to how to address expansive soil foundation problems.  One essential element is to seek the council of an unbiased structural engineer who specializes in this area.

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How much damage should a homebuyer be willing to tolerate? 

Structural safety problems that result from foundation movement are clearly intolerable.  But beyond that, the correct answer is that there is no one answer that is correct for everyone.  Different people have a different tolerance for cracks in wall coverings and sticking doors.  It is important in searching for a home to purchase that  you be realistic. Demanding a house that shows no signs of foundation movement, has never shown any sign of foundation movement and never will show damage that could be attributable to foundation movement is not realistic.  You should not buy any home that you are not comfortable with, especially without having a structural engineer make a damage evaluation.  But the ultimate decision as to how much damage you are willing to tolerate is one only you can make.

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How far out-of-level is acceptable? 

There is no answer to this question that universally accepted.  The Post-Tensioning Institute has published a peer-reviewed paper in which is it is stated that a diagnosis of excessive expansive soil movement cannot be made unless the slab surface is out of level substantially in excess of published American Concrete Institute (ACI) standardized construction levelness tolerances for slab-on-ground foundation construction.  The ACI publishes several different construction tolerances but recommends the use of what are called F-numbers.  The F-number system allows the elevation of two points 10-feet apart to be different by as much as 1.25 inches.  If a foundation were to deflect L/360 in both directions (which most engineers would consider acceptable), the resulting slope (adding an as-constructed slope to the slope caused by foundation distortion) could result in a foundation surface slope of 1.65 inches or more over 10-feet.  A slope greater than 1% (1.2 inches over 10-feet) is noticeable by most people.  Thus, a noticeable floor slope may or may not indicate excessive foundation movement.  You should also understand that the as-constructed slope and the slope due to foundation movement may not add together; the foundation may distort in a way that makes the slab surface more level, not less level.

Some engineers prefer to judge the levelness of the foundation due to distortion by looking, not at the levelness of the slab surface, but at the levelness of first floor counter-tops and window stools since these elements are normally constructed to much tighter levelness tolerances than slab-on-ground foundation surface tolerances.  If the countertops and stools are reasonably level within normal construction tolerances, then it is reasonable that any floor out-of-levelness is probably due to original construction.

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Will my house fall down? 

It is important to understand that it is extremely unlikely for expansive soil foundation movement to cause a house to collapse.  First very few houses collapse for any reason. The most common reason for this type of failure is probably fire.  It is conceivable for a house that has extremely severe moisture and termite damage to collapse, especially if it is abandoned.  Houses under construction have been known to collapse when subjected to high winds.  Houses under construction are subject to this risk because key structural elements have not yet been installed.  Of course, extreme weather events such as hurricanes and tornadoes can also cause a house structure to collapse.  But I do not know of a single case in which expansive soil movement caused a house structure to collapse.

What seasonal weather-related expansive soil movement can do is cause cosmetic damage to the house in various forms, usually drywall cracking and brick veneer cracking.  Door frames can become distorted so that doors no longer fit properly in their frame; also, doors may not latch and could stick and bind.  It is also possible for foundation movement to cause framing members to pull apart to some degree.  In most cases, the damage is restricted to cosmetic damage that can be repaired using normal decorative repair techniques or minor functional problems such as sticking doors that can be corrected by adjusting or reinstalling the door.

The fear of a house falling down due to expansive soil movement is not based on reality.

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